Conventionally, most of the packages for semiconductor devices consisted of a ceramic pin grid array (CPGA) or a plastic pin grid array (PPGA). The CPGA is relatively expensive while the PPGA is known to be relatively less reliable and poor in its capability to dissipate heat. For these reasons, the metal pin grid array (MPGA) and the metal ball grid array (MBGA) are being preferred. They are based on the use of a metal base and a metal cover, and are considered to be equally reliable as the CPGA, and have an even better heat dissipating capability than the CPGA. Furthermore, the MPGA and the can be made as economical as the PPGA by suitably selecting the materials for the package. In the MPGA and the MBGA, a semiconductor chip is normally placed on the metal base, and an annular circuit board provided with a circuit pattern is placed around the semiconductor chip. The pads provided on the semiconductor chip are connected to the associated pads of the circuit board by bonding wires.
As the density of semiconductor integration is progressively increased in recent years, the density of the circuit pattern of the circuit board has to be accordingly increased, and this leads to the increase in the line length of the electroconductive strips of the circuit pattern, and the reduction in the pattern width and the gap between adjacent electroconductive strips. Therefore, degradation in high frequency properties or an increase in the signal time delays, and an increase in the noise level, resulting from such a trend, can become a problem. Furthermore, the progressively increasing clock speed of the CPU and other semiconductor devices even further compounds such a problem.
Another important problem in the MPGA and the MBGA is the precision in the positioning of the semiconductor chip on the metal base. It is conceivable to place a buffer layer made of ceramic material such as alumina Al.sub.2 O.sub.3, or molybdenum (Mo) between the semiconductor chip and the metal base. These materials have thermal expansion coefficients which are comparable to that of silicon (alumina 7.3 .mu./.degree.C. molybdenum 5.0 .mu./.degree.C., and silicon 3.5 .mu./.degree.C.). However, the number of component parts has to be undesirably increased, and the overall structure becomes undesirably complex. As a result, the cost is increased, and the heat dissipating capability is diminished.
It is also conceivable to use CuW and AlN, having the thermal expansion coefficients of 6.0 .mu./.degree.C. and 4.9 .mu./.degree.C., respectively, for the base as they have a favorable heat dissipating capability and their thermal expansion coefficients are comparable to that of silicon. However, these materials are too costly for practical use. CuW cost about 120 times and AlN cost about 260 times more than Al.
These problems also apply to those semiconductor devices mounted on a metallic circuit board. The following table compares the properties and costs of various materials.
TABLE 1 ______________________________________ coefficients coefficients of thermal of thermal conduction expansion specific cost materials (W/m.degree. .multidot. K) (.mu./.degree.C.) weight (*1) ______________________________________ metals Al 237 23.0 2.7 1 Cu 398 16.0 8.9 3.2 Fe 80 12.0 7.9 0.4 CuW 210 6.0 17.0 120 ceramics Al.sub.2 O.sub.3 20 7.3 3.8 80 AlN 170 4.9 3.3 260 resin epoxy + 0.3 15-17 1.85 0.2 glass semiconductor Si 150 3.5 2.3 -- ______________________________________ (*1) As compared to the cost of Al.
In the MPGA and the MBGA, the semiconductor chip is typically attached to the metal base by using solder or an epoxy bonding agent. When economical metal material such as aluminum is used, although it has a favorable heat dissipating property, because of the large difference in the thermal expansion coefficients of the semiconductor chip made of silicon (with the thermal expansion coefficient of 3.5 .mu./.degree.C.) and the metal base made of aluminum (with the thermal expansion coefficient of 23 .mu./.degree.C.), the semiconductor chip is subjected to a significant thermal stress. As a result, the integrated circuit in the semiconductor chip may fail to operate properly, or even cracks may develop in the semiconductor chip. This can seriously impair the reliability of the semiconductor chip.